This invention relates to a method of and an apparatus for assessing the insulation condition in electrical equipment, cables, etc. The present invention is to be utilized for detecting any deterioration in insulation in electrical equipment and cables by detecting the corona discharge or the partial discharge generated when such a deterioration occurs.
Generally, various factors can locally cause a poor insulation in an underground cable and the electrical equipment connected thereto.
At least 80 percent of serious accidents in such cables and their equipment are attributable to such a deterioration in insulation which can be caused by external mechanical forces, chemical changes in the insulators, tree-branch-like deterioration of the insulator known as "water tree", etc. In view of this, various methods of assessing insulation conditions have been proposed.
According to one of these methods, a power supply system is periodically tested for the insulation condition thereof by temporarily stopping the power supply. In this method, the test may be conducted, for example, by applying DC voltage to the power line. What is to be measured to assess the insulating performance of the system is: Firstly, partial discharge; secondly, residual voltage, discharge current or residual charge, which shows reduction in dielectric strength; or potential damping and leakage current.
On the other hand, there is a method of assessing the insulating performance of a power line by applying AC voltage to the power line concerned. What is to be measured is: partial discharge; or dielectric relaxation which can be assessed through dielectric tangent.
Apart from this, there is a method wherein a power supply system is examined in a hot-line condition using a portable measuring apparatus.
In the above-described method of assessing the insulataing performance of a power supply system by periodically stopping the power supply, the power lines forming the system have to be examined one by one, so that it takes a lot of time to conduct the measurement. Further, there is a limitation in the number of places that can be measured during a single stopping of power supply, so that if the insulation of some part has gradually deteriorated, there is no knowing the trend thereof. Accordingly, no preventive measures can be taken against a deterioration of this type.
On the other hand, the problem with the method in which a power supply system is examined in a hot-line condition using a portable measuring apparatus is that only a skilled engineer can perform the measurement because of the difficulty in securing the operational safety, to say nothing of the labor intensiveness in the preparatory operations and the measurement.